The future international standards relating to the control of the emissions of polluting gases produced by cars (the so-called “Euro5” and “Euro6” or “Tier2 Bin5” standards) provide for a very low limit for the NOx molecules which may be released in the atmosphere.
Respecting such limits is particularly important especially for diesel engines; for this reason, it has been suggested to provide the exhaust system of a diesel engine with an additional NOx Selective Catalytic Reduction (SCR) converter adapted to convert the molecules of NOx (NO2 or NO) to nitrogen (N2), which is an inert gas, and water (H2O). The reduction reaction of the molecules of NOx to nitrogen (N) is complicated to obtain without the use of an appropriate reducing agent which has generally been identified as ammonia (NH3). The reducing agent must be injected into the exhaust system and upstream of the SCR catalytic converter so as to mix with the exhaust gases before entering the SCR catalytic converter.
However, for obvious safety reasons connected to the toxicity of ammonia, is not recommendable to store ammonia within a car. Therefore, it has been suggested to store and inject an aqueous solution of urea, as urea decomposes to ammonia by the effect of the heat of the exhaust gases and partly also by catalytic effect.
To perform the injection of the aqueous solution of urea upstream of the SCR catalytic converter, it has been suggested to use an electromagnetic injector by all means similar to the electromagnetic injectors currently used for the injection of fuel in internal combustion engines. In this manner, existing components with a proven efficiency and reliability may be used and there is therefore no need to develop new components with obvious saving of time and costs. However, mounting in an exhaust conduit arranged immediately upstream an SCR catalytic converter of an electromagnetic injector originally made for the injection of fuel has revealed problematic, as the temperatures within the exhaust conduit are very high (from a minimum of about 150° C. to a maximum of about 900° C. during the regeneration steps) and may produce an excessive overheating of the electromagnetic injector with the subsequent destruction of the electromagnetic injector itself. By way of example, the temperature of the outer jacket of a currently produced electromagnetic injector must not exceed about 150° C. to avoid melting the insulation of the copper conductor forming the electromagnet coil, while the nose of a currently produced electromagnetic injector must not exceed about 250° C. to avoid steel annealing phenomena which could alter the mechanical features thereof.
Furthermore, it must be noted that not only is it required to ensure that the electromagnetic injector is appropriately thermally insulated from the heat of the exhaust system, but it is also required to ensure that the heat, which has been transmitted by conduction and radiation to the injector and the heat produced by Joule effect by the electromagnet coil within the electromagnetic injector, is effectively dissipated at the same time.